1. Field of the Inventive Concept
Embodiments relate to a method and apparatus for measuring an overlay.
2. Description of the Prior Art
With the rapid spread of information mediums such as a computer, semiconductor devices have made rapid progress. Semiconductor device are serve to enable a device at high speed but also to provide a large-scale storage capacity. To answer such requirements, a technique of fabricating a semiconductor device has been developed in the direction of improving the integrity, reliability, and response speed of the semiconductor device.
Accordingly, as a scheme for increasing the competitiveness in the semiconductor industry, unit processes that can secure high product yield have been developed, and a method and apparatus for measuring process errors in the respective unit processes have been actively researched. Particularly, in the case of a photolithography process, which is one of the core elements of a semiconductor fabricating processes, the change of process conditions is frequent. Thus, process development for coping with changes in processing conditions and an apparatus for performing the same are necessary.
In the photolithography process, misalignment of a photoresist pattern that is formed by exposing and developing the photoresist should be considered. Misalignment is caused by a reduction of an alignment margin due to high integration of a semiconductor device, large diameter of a wafer, and increase in the photolithography process. Thus, it becomes more difficult to achieve an accurate align. In order to prevent misalignment, it is essentially required to optimize the overlay measurement that is a task for confirming the alignment of the photoresist pattern that is formed on a wafer.
An IBO (Image Based Overlay) measurement method measures the overlay by detecting the center coordinates of a vernier and the center coordinates of a main beam and measuring the relative difference between two coordinates. The image based overlay measurement method meets with several technical limitations as design rules become smaller.
On the other hand, a DBO (Diffraction Based Overlay) measurement method uses a main beam and a vernier which have some offsets as measurement keys. Since the vernier is arranged on the main beam, the main beam and the vernier overlap each other in space. Particularly, since the vernier is arranged on the main beam, the state of the main beam exerts an influence on the state of the vernier. For example, if the step height of the main beam is great, coating inferiority or undercut may occur when the vernier is formed. Further, the pattern of the vernier may be destroyed. In this case, the accuracy of the DBO measurement method is lowered.